A - 小C语言—词法分析程序

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#include<bits/stdc++.h>
using namespace std;
string kind[5] = { "keyword","identifier","integer","boundary","operator" };
string key[6] = { "main","if","else","for","while","int" };
void Check(string token){
    if(token[0]>='0'&&token[0]<='9'){
        cout<<"("<<kind[2]<<","<<token<<")"<<endl;
        return;
    }
    else{
        for(int i=0;i<6;i++){
            if(token==key[i]){
                cout<<"("<<kind[0]<<","<<token<<")"<<endl;
                return;
            }
        }
        cout<<"("<<kind[1]<<","<<token<<")"<<endl;
    }
}
int main(){
    string str;
    while(cin>>str){
        string token="";
        int len = str.length();
        for(int i=0;i<len;i++){
            //遇到界符，往往表示单词结束，判断
            if(str[i]=='('||str[i] == ')'||str[i]=='{'||str[i]=='}'||str[i]==','||str[i]==';'){
                if(token.length()){
                    Check(token);
                }
                cout<<"("<<kind[3]<<","<<str[i]<<")"<<endl;
                token="";
            }//遇到运算符，往往表示单词结束，判断是否为双目运算符，是则输出，否则单目运算符，输出单目运算符
            else if(str[i]=='+'||str[i]=='-'||str[i]=='*'||str[i]=='/'||str[i]=='='||str[i]=='>'||str[i]=='<'||str[i]=='!'){
                if(token.length()){
                    Check(token);
                }
                token="";
                if(i+1<len&&str[i+1]=='='){
                    cout<<"("<<kind[4]<<","<<str[i]<<"="<<")"<<endl;
                    i++;
                }
                else{
                    cout<<"("<<kind[4]<<","<<str[i]<<")"<<endl;
                }
            }//该字符不是界符也不是运算符，加入token中
            else{
                token+=str[i];
            }
            
        }//每次循环结束，判断token是否为空，不为空则检查其类型
        if(token.length()){
            Check(token);
        }
    }
}

B - 识别浮点常量问题

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#include<bits/stdc++.h>
using namespace std;
bool isNum(char c){
    if(c>='0'&&c<='9')
        return true;
    return false;
}
int findP(int l, int r, string s){
    for(int i=0;i<=r;i++){
        if(s[i]=='.'){
            return i;
        }
    }
    return -1;   
}
int countP(int l, int r, string s){
    int cnt=0;
    for(int i=l;i<=r;i++){
        if(s[i]=='.'){
            cnt++;
        }
    }
    return cnt;       
}
bool isSign(char c){
    if(c=='+'||c=='-')
        return true;
    return false;
}
bool isSmallNum(int l, int r, string s){
    int p_l=findP(l,r,s);
    int flag=0;
    //判断小数点个数
	if(!(countP(l,r,s)==1)){
		return false;
	} 
	//判小数点左侧是否有数字
	for(int i=0;i<p_l;i++){
		if(isNum(s[i])){
			flag=1;
			break;
		}
	}
	if(!flag) return false;
	flag=0;
	//判小数点右侧是否有数字
	for(int i=p_l+1;i<=r;i++){
		if(isNum(s[i])){
			flag=1;
			break;
		}
	}
	if(!flag) return false;
	for(int i=l;i<=r;i++){
		if(i==l&&isSign(s[l])){
			if(r-l+1==1) return false;
			continue;
		}
		else if(s[i]=='.'){
			continue;
		}
		else if(isNum(s[i])){
			continue;
		}
		else{
			return false;
		}
	}
	return true;
	 
}

int FindE(string s){
    for(int i=0;i<s.size();i++){
        if(s[i]=='e'||s[i]=='E'){
            return i;
        }
    }
    return -1;
}
bool isInt(int l, int r, string s){
        for(int i=l;i<=r;i++){
            if(i==l&&isSign(s[l])){
            	if(r-l+1==1) return false;
                continue;
            }
            else if(isNum(s[i])){
                continue;
            }
            else{
                return false;;               
            }
    }
    return true;
}
int main(){
    string s;
    int flag=0;
    getline(cin, s);
    int l=0, r=s.size()-1;
    while(s[l]==' ') l++;
    while(s[r]==' ') r--;
    int len=s.size();
    //3e-.3
    int res=FindE(s);
    if(res==-1){
        if(isSmallNum(l, r, s)){
            cout<<"YES";
            return 0;
        }
        else{

            cout<<"NO";
            return 0;
        }

    }
    else{
        if(res==l||res==r){
            flag=1;
        }
        if(!isSmallNum(l, res-1, s)&&!isInt(l,res-1,s)){
            flag=1;
        }
        if(isInt(res+1,r,s)){
            ;
        }
        else{
            flag=1;
        }
        if(flag==0){
            cout<<"YES";
            return 0;
        }
        else{
            cout<<"NO";
            return 0;
        }
        return 0;        
    }
    
    
}